A programmable logic controller (PLC) is communicably connectable to an external instrument, e.g., a display device, a computer, or another PLC, so that it can receive a write request command from the external instrument. In accordance with a write request from an external instrument, the PLC may execute the process of writing to a data area in which computation data are stored. The PLC needs to protect important computation data stored in the data area from being falsified by a malicious third party or being rewritten due to an erroneous operation.
Patent Literature 1 discloses a PLC-related technique for setting, in a memory for storing computation data, an area in which writing of data associated with a request from. an external instrument is prohibited. The PLC can protect computation data stored in the write-prohibted area. According to the technique of Patent Literature 1, the write-prohibited area is designated using the address of the memory.